1. Field of Invention
The present invention relates to a fluorocarbon resin-coated member suitable for a heating belt, a heating roll (including a pressurizing belt and a pressurizing roll) and the like for use in a fixing device; a manufacturing method thereof, a fixing device of sing a powder toner image formed on a support by applying heat and pressure simultaneously so as to fix the image; and an image forming apparatus which uses the fixing device.
2. Description of the Related Art
Conventional apparatuses which use an electrophotographic process, such as copying machines, require fusing unfixed toner images formed on recording sheet so as to form the unfixed toner images into a permanent image. Methods that are known as the fixing method include a solvent fixing method, a pressure fixing method, and a thermal-fixing method. The solvent-fixing method has a defect of frequent occurrence of odor and hygienic problems caused by evaporation of solvents. On the other hand, the pressure-fixing method also has a defect that the fixing efficiency thereof is lower than that of other fixing methods. Thus, both methods have not been widely commercialized. Instead, the thermal-fixing method of fusing the toner on the recording sheet (recording medium) generally by heating is used widely in fixing an unfixed toner image.
A typical example of conventionally-known thermal-fixing devices for use in the thermal-fixing method is a device which uses a thermal-fixing roll system The device has a heating roll which has a cylindrical metal core having a beater lamp inside thereof and a heat-resistant releasing layer on an external surface thereof, and a pressurizing roll having a heat-resistant elastomer layer formed on the peripheral surface of another cylindrical metal core placed in contact with the heating roll (fixing roll). An unfixed toner image formed on a supporting material such as plain paper is conveyed into and fixed between these rolls under a pressure of approximately 1 to 15 kg/cm2, preferably approximately 3 to 10 kg/cm2. The heating roll-fixing device used in this system is more widely used because it is superior in energy conservation and speed because of its thermal efficiency and free from fire hazards due to paper clogging in comparison to other thermal-fixing devices such as hot air-fixing or oven-fixing processes.
In order to satisfy the recent need for further acceleration of fixing in such a heating roll-fixing device, it is necessary to expand the width of the nip region, i.e., nip width, to match with the increase in fixing speed. Methods for expanding the nip width include increasing the load between rolls, increasing the thickness of an elastic layer of a fixing roll, expanding diameters of a fixing roll and a pressurizing roll, and the like. However, there are limitations to the fixing speed which can be made achieved by these methods, and thus a heating roll-and-belt firing device and a heating belt-and-roll fixing device have been developed for use in yet faster fixing ranges.
The fixing components (rolls and belts) for use in the heating roll-and-belt fixing device or heating belt-and-roll fixing device are roughly divided into two groups: 1) silicone rubber- or fluorine rubber-coated components in which a silicone or fluorine rubber is thinly coated on a base material via a primer: and 2) fluorocarbon resin-coated components in which a fluorocarbon resin such as a copolymer of tetrafluoroethylene and a perfluoroalkylvinylether (herein referred to as “PFA”) or polytetrafluoroethylene (hereinafter referred to as “PTFE”) is coated on a base material via a primer.
Among these fixing components, the silicone rubber-coated component contains silicone oils called free oils inside the materials thereof and these oils exert a significant influence in terms of releasing characteristics, and a fixing component containing the free oils in a greater amount shows better releasing characteristics. However, presence of the free oils causes problems of deterioration in rubber strength and deformation of the belt due to emission of the free oils.
On the other hand, fluorine rubber-coated components, which are very solid, higher in abrasion wearing resistance, and elastic, are effective in providing an image higher in quality. However, the fluorine rubber, which inherently repels the polydimethylsiloxane oil (silicone oil) commonly used as a release oil, is less effective in forming a releasing layer of the oil at the interface with the toner image. Thus, the combination of a fluorine rubber and a dimethylsiloxane oil (silicone oil) cannot be used when a low-melting point, high-coloring toner such as color toner is used, because it is inferior in releasing characteristics. A method of using a modified polydimethylsiloxane oil in which part of the polydimethylsiloxane oil is modified with a mercapto group —SH or an ammo group —NH2 has been proposed to overcome the problem. In such a case, the functional group such as mercapto or amino reacts with the metal oxides (e.g., MgO, PbO, or the like) and the double bonds contained in the fluorine rubber, allowing formation of a thin silicone oil film at the molecular level on the belt surface, which functions as a releasing layer and modifies the fixing-component surface into a surface superior in releasing characteristics.
However, the silicone oil, which is effective in modifying the surface into a high-release surface, also makes the surface of a copying sheet or, during double-faced copying, a paper-supplying roll highly releasable at the same time, and thus, causes the problems, for example, that it is difficult to adhere a stationery tape or the like on a copying sheet obtained or to supply paper smoothly with the paper-supplying roll. If the amount of the modified silicone oil used is reduced or almost no modified silicone oil is used for prevention of such problems, the belt surface exhibits the rubbery stickiness inherent to the fluorine rubber and may cause the problem that the surfaces of the copier paper and the fluorine rubber-coated belt adhere to each other during fusion, prohibiting release of the copier paper from the fluorine rubber surface after fixing he problem becomes amplified when the copier paper is a coated paper higher in surface smoothness and, in particular, when the surface thereof is treated with a coating agent.
For prevention of generation of the problems above, there exists a need for a fixing component having a surface that is less sticky with respect to the copier paper and allowing more efficient release thereof, even during fixing when the amount of the modified silicone oil used is reduced or almost no modified silicone oil is used. There is a requirement for the firing component that the adhesiveness between toner and fixing component is lower and the adhesiveness between copier paper and fixing component surface is lower even when the input of modified silicone oil during fusion is low.
For satisfying these requirements, a fluorocarbon resin material has come to be used more frequently as the surface layer material formed on the fixing component surface. Examples of the fluorocarbon resin materials formed on the fixing component surface include fluorocarbon resin tube materials prepared by beat-melting and extruding a fluorocarbon resin and fluorocarbon resin-coated materials prepared by coating, drying, and sintering a coating material containing a fluorocarbon resin powder dispersed in a solution on a base material.
The fluorocarbon resin tube material and the fluorocarbon resin-coated aerial have respective advantages and disadvantages. The fluorocarbon resin tube material has an advantage that the material properties thereof are similar to bulk fluorocarbon resin (i.e., the abrasion wearing resistance and the like are superior), but also has a disadvantage that it is difficult to process it into various shapes because of constraints in processing methods.
On the other hand, the fluorocarbon resin-coated material has an advantage that it is possible to process it into various shapes easily by coating technology, but also has a disadvantage that the material properties thereof, in particular abrasion wearing resistance, are inferior to those of bulk fluorocarbon resin because it is a sintered powder.
On the other hand, as described above, there currently exists a need recently for further increase in the fixing speed of fixing devices, and to satisfy this need, it is necessary to expand the width of nip region, i.e., nip width, to match the increase in fixing speed. Methods of increasing the load applied between rolls, thickening the elastic layer of fixing roll, and increasing the diameters of the fixing and pressurizing rolls, and also heating roll-and-belt-fixing or heat belt/roll-fixing devices are under development aimed at expanding the nip width. When a fixing device is further accelerated, the stress applied to the fixing components also increases. In particular, the abrasion stress applied to the fluorocarbon resin material, the surface layer material of the fixing component, increases significantly. It is obvious that use of a fluorocarbon resin-coated material as the surface layer material of the fixing component under such high stress conditions results in the problem of frequent trouble due to abrasion of the surface layer material.
A method of smoothing the fluorocarbon resin-coated surface at normal temperature with a pressurizing roll and then heat-treating the fluorocarbon resin-coated surface (see, for example, Japanese Patent Application Publication (JP-B) No. 3-80277) and a method of softening the fluorocarbon resin-coated surface at 250 to 300° C. and smoothing it with a pressurizing roll (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-109529) we, for example, known as methods to overcome this problem; but because the treatment temperature is lower than the melting point of the fluorocarbon resin, these methods still have the problems that it is impossible to raise the density of the fluorocarbon resin-coated layer sufficiently and to apply a pressure uniformly to the fixing component surface during pressurization with a pressurizing roll.
Also known are a method of bringing the fluorocarbon resin-coated surface into contact with a heater previously heated to a temperature of not lower than the melting point under pressure and heating the surface while rotating the heater surface or the coated surface (see, for example, JP-B No. 7-43556) and a method of coating a fluorocarbon resin powder on a cylindrical base material, inserting the base material in another cylinder having an inner diameter slightly larger than the external diameter of the base material, and applying heat and pressure by using the difference in thermal expansion coefficients thereof (see, for example, JP-A No. 11-5059 or 2001-277266); but these methods also had the problem that it was not possible to raise the density of the fluorocarbon resin-coated layer uniformly, because it was not possible to apply pressure uniformly to the fixing component surface during pressurization by using the pressurizing roll and the pressurizing member (cylinder).